PLL is a loop control system. The phase difference between the benchmark frequency and the output frequency generates control signals for the VCO, thus regulating the output frequency to the expected value. As an important part of a PLL, a VCO is configured to generate clock signals. FIG. 1 shows a common chip integrated VCO circuit. L, C1, and C2 construct the resonance loop, whereas C1 and C2 are capacitors. MN1, MN2, and MP2 construct two pairs of negative resistance tubes. MP3 is the current source tube. MP1, MP2, and MP3 are Positive Metal Oxide Semiconductor (PMOS) tubes, while MN1 and MN2 are Negative Metal Oxide Semiconductor (NMOS) tubes. The signal input point VCTRL of the VCO is located at the middle of the distance between C1 and C2. Oscillation signals of the VCO are sent to both sides of L. The oscillation frequency of the VCO: f=1/√{square root over (LC)}, whereas L indicates the inductance of the resonance loop, and C indicates the capacitance of the resonance loop. In the actual circuit, L and C are functions of the voltage and temperature of the power supply. That is, when the voltage and temperature of the power supply change, the oscillation frequency of the related VCO changes.
After analyzing the existing technologies, the inventor finds that the oscillation frequency of the VCO increases or decreases with the change to the voltage or temperature of the circuit in actual applications. For example, the voltage or temperature of the power supply in a system changes after the system runs for a period of time. In this case, the oscillation frequency of the VCO changes accordingly. PLL is a closed loop system. When the voltage and temperature of the power supply undergo radical changes, the oscillation frequency of the VCO varies in a broad range, and thus the PLL is out of lock. When the rate at which the voltage and temperature change is low, the negative feedback of the PLL automatically regulates the input voltage of the VCO to maintain the output frequency. In this case, however, the range of the input voltage of the VCO is enlarged, increasing the difficulty in circuit design. From another perspective, when the input voltage range of the VCO is unchanged, the oscillation frequency range of the VCO is narrowed due to impact of the voltage or temperature.